RSV and HMPV are cytoplasmic enveloped RNA viruses of the paramyxovirus family. Their genomes are single strands of negative-sense RNA of 15.2 kb (RSV) or 13.3 kb (HMPV) that encode 10 mRNAs and 11 unique proteins (RSV) or 8 mRNAs and 9 unique proteins (HMPV). Each virus encodes a nucleoprotein N, phosphoprotein P, matrix protein M, small hydrophobic protein SH, major glycoprotein G, fusion glycoprotein F, polymerase factors M2-1 and M2-2, and the polymerase protein L. In addition, RSV encodes two nonstructural proteins NS1 and NS2. We evaluated the strategy of codon-pair deoptimization (CPD) as a means of developing genetically and phenotypically stable attenuated RSV strains. It is well known that there is a bias in codon-pair usage in nature. Specifically, any given pair of amino acids in a polypeptide chain has the possibility to be encoded by a variety of different combinations of synonymous codons due to the degeneracy of the genetic code, but the observed usage of codon-pairs typically is biased to favor a subset of the possible combinations. One factor in this bias is thought to be translational efficiency and accuracy, because certain combinations of tRNA pairs are favored at the A and P sites in the ribosome due to tRNA geometry and other factors. CPD involves the deliberate introduction of under-represented synonymous codon-pairs into numerous sites in protein-coding sequence to achieve sub-optimal expression. These substitutions only involve the ORFS, and thus non-protein-coding genome regions are not affected. Also, CPD involves only synonymous codon substitutions, and thus amino acid coding is unaffected. In addition, CPD applied to one or several genes typically involves hundreds or thousands of nucleotide changes, and thus should be highly refractory to de-attenuation. Recently, CPD was applied to poliovirus and influenza virus and was shown to result in attenuated strains. We designed the following set of four CPD RSV genomes in which the indicated ORFs were recoded: (i) Min A; NS1, NS2, N, P, M, and SH (i.e., the left-hand third of the genome); (ii) Min B; G and F (located in the middle of the genome); (iii) Min L; L (located at the right-hand end of the genome); and (iv) Min FLC; all ORFs except M2-1 and M2-2. The recoded genome regions were synthesized commercially and the four CPD viruses were constructed and recovered by reverse genetics. All of the CPD viruses were temperature-sensitive (level of sensitivity: Min FLC>Min L>Min B>Min A) for replication in vitro. We speculate that CDP may slow down the rate of translation sufficiently to create protein-folding problems that are exacerbated by increased temperature. All of the CPD mutants grew less efficiently in vitro than wild type (wt) RSV, even at the permissive temperature of 32C (growth efficiency: wt>Min L>Min A>Min FLC>Min B). Thus, CPD of G and F ORFs provided the greatest effect. The CPD viruses exhibited a range of restriction in mice and African Green Monkeys (AGM) and induced immunity against wt RSV. This study identified new vaccine candidates for RSV and showed that CPD of a nonsegmented negative-strand RNA virus can rapidly generate vaccine candidates with a range of attenuation phenotypes. We used gene-deletion HMPV strains to evaluate the role of the attachment G and small hydrophobic SH glycoproteins on HMPV uptake by primary human monocyte-derived dendritic cells (MDDC) in vitro, and on subsequent MDDC maturation and activation of autologous T cells. Deletion of G and SH (delSHG) conferred increased infectivity but had little effect on MDDC maturation. However, MDDC stimulated with &#916;SHG induced increased proliferation of autologous Th1-polarized CD4+ T cells. This effect was independent of virus replication. Increased T cell proliferation was strictly dependent on contact between virus-stimulated MDDC and CD4+ T cells. Confocal microscopy revealed that deletion of SH and G was associated with an increased number of immunological synapses between memory CD4+ T cells and virus-stimulated MDDC. Uptake of HMPV by MDDC was found to be primarily by macropinocytosis. Uptake of wild-type (WT) virus was reduced compared to &#8710;SHG, indicative of inhibition by the SH and G glycoproteins. In addition, DC-SIGN-mediated endocytosis provided a minor alternative pathway that depended on SH and/or G and thus operated only for WT. Altogether our results show that SH and G glycoproteins reduce the ability of HMPV to be internalized by MDDC, resulting in a reduced ability of the HMPV-stimulated MDDC to activate CD4+ T cells. This study describes a previously unknown mechanism of virus immune evasion. This is of interest because reinfection by HMPV is common throughout life without need for significant antigenic change, suggesting that protective immune response to HMPV is incomplete and short-lived. The present study provides a mechanism that might contribute to suppressing host immunity.